As described more fully in U.S. Pat. No. 4,453,723, there is a need in aircraft for a fire resistant seal which has a garter spring movably mounted in a centrally disposed bore in the seal where the bore and the garter spring are in spaced relationship to the inner work contacting surface of the seal and where the diameter of the turns of the garter spring is slightly less than the diameter of the cross section of the bore. With this arrangement, when the garter spring is in the bore the garter spring can contract independantly of the ring shaped seal over a wide temperature range for a prolonged period of time. Additionally, with the diameter of the turns of the garter spring only slightly smaller than the diameter of the cross section of the bore in the seal the compressive force exerted by the garter spring will be spread over a larger surface of the work engaging part of the seal resulting in an extended work life.
The seals described above and more fully in U.S. Pat. No. 4,453,723, are used to connect aircraft ducts together. Since these ducts often conduct air to the passenger compartment, these seals must have sufficient mass to withstand high temperatures for a reasonable length of time, to prevent smoke which may be caused by a fire, from entering the ducts and getting into the passenger compartment. Moreover, the garter spring inside seal ring must be fully enclosed to prevent high temperatures such as may be caused by a fire, from destroying the garter spring and rendering the seal ineffective. This is because a garter spring loses elasticity at high temperatures, such as may be caused by a fire.
However, the previous method of forming a seal, as described in U.S. Pat. No. 4,453,723 was unsatisfactory because the tolerances and physical characteristics of the finished seals were not sufficiently uniform.
This prior method required a tubular semi-cured rubber or elastomeric insert to be formed by a conventional extruding machine. This insert was cut to the required length. Next a garter spring, also cut to the desired length was inserted in the bore of the tube. The ends of the garter spring were designed so they could be secured together in a manner well known in the art to form a tubular ring with the garter spring movably mounted in the bore of the tubular ring.
Next layers of silicon impregnated fiberglass cloth were inserted in a mold cavity. Then the ring shaped tubular insert was laid over the fiberglass cloth in mold cavity, and the mold was closed. Finally the mold was inserted in an oven long enough to cure the elastomeric material and form the seal.
This method was not satisfactory because in the curing process, contrary to expectation, the semi-cured elastomeric insert would contract down around the garter spring in a random fashion so that the garter spring was not always free to move in the seal. Consequently, seals manufactured by this method did not have uniform performance characteristics and were not reliable.
To overcome this problem it is proposed to eliminate the problems caused by using a tube of a elastomeric material in a semi-cured state and instead use a tube of a elastomeric material in a set-cured condition. The ends of the tube would be cemented together to form a ring and a slit would be formed in the tube which communicates with the bore in the tube. Then the facing surfaces of the slit would be pried apart and a suitably shaped garter spring would be inserted in the tube. After this the facing sides of the slit would be cemented together to isolate the garter spring in the bore in the tubular ring.
However the critical importance of forming the slit so the facing surfaces of the slit complement each other was not initially realized. Because of this, attempts were made to extrude the tube of elastomeric material in a set-cured condition with a slit formed in the tube by the extruding machine. However this approach was not satifactory because when the facing surfaces of the slit were cemented together with the garter spring in the bore, the shape of the seal was distorted so the seal did not have uniform performance characteristics. Thus the seals were not reliable and were no better than the seals produced in accordance with the teachings in U.S. Pat. No. 4,453,723.
What is needed therefore, and comprises an important object of this invention, is to provide a method of forming a seal with a garter spring movably mounted within a bore in the seal by cutting a slit in the periphery of the seal for insertion of the garter spring into the bore of the seal, in such a way that the facing surfaces of the slit are complementary to each other. With this arrangement when the facing surfaces of the slit are cemented together with the garter spring in the bore of the seal, the shape of the seal will not be distorted, and seals made this way will have uniform performance characteristics.